# std::ranges::reverse_copy, std::ranges::reverse_copy_result

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::bidirectional_iterator I, std::sentinel_for<I> S, std::weakly_incrementable O > requires std::indirectly_copyable<I, O> constexpr reverse_copy_result<I, O> reverse_copy( I first, S last, O result ); | (1) | (since C++20)
template< ranges::bidirectional_range R, std::weakly_incrementable O > requires std::indirectly_copyable<ranges::iterator_t<R>, O> constexpr reverse_copy_result<ranges::borrowed_iterator_t<R>, O> reverse_copy( R&& r, O result ); | (2) | (since C++20)
Helper types |  |
template< class I, class O > using reverse_copy_result = ranges::in_out_result<I, O>; | (3) | (since C++20)

1) Copies the elements from the source range `[`first`,` last`)` to the destination range `[`result`,` result + N`)`, where `N` is ranges::distance(first, last), in such a way that the elements in the new range are in reverse order. Behaves as if by executing the assignment *(result + N - 1 - i) = *(first + i) once for each integer `i` in `[`​0​`,` N`)`. The behavior is undefined if the source and destination ranges overlap.
2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first, last | - | the range of elements to copy
r | - | the range of elements to copy
result | - | the beginning of the destination range.

### Return value

{last, result + N}.

### Complexity

Exactly `N` assignments.

### Notes

Implementations (e.g. MSVC STL) may enable vectorization when the both iterator types model `contiguous_iterator` and have the same value type, and the value type is TriviallyCopyable.

### Possible implementation

See also the implementations in MSVC STL and libstdc++.

    struct reverse_copy_fn
    {
        template<std::bidirectional_iterator I, std::sentinel_for<I> S,
                 std::weakly_incrementable O>
        requires std::indirectly_copyable<I, O>
        constexpr ranges::reverse_copy_result<I, O>
            operator()(I first, S last, O result) const
        {
            auto ret = ranges::next(first, last);
            for (; last != first; *result = *--last, ++result);
            return {std::move(ret), std::move(result)};
        }

        template<ranges::bidirectional_range R, std::weakly_incrementable O>
        requires std::indirectly_copyable<ranges::iterator_t<R>, O>
        constexpr ranges::reverse_copy_result<ranges::borrowed_iterator_t<R>, O>
            operator()(R&& r, O result) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), std::move(result));
        }
    };

    inline constexpr reverse_copy_fn reverse_copy {};

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <string>

    int main()
    {
        std::string x {"12345"}, y(x.size(), ' ');
        std::cout << x << " → ";
        std::ranges::reverse_copy(x.begin(), x.end(), y.begin());
        std::cout << y << " → ";
        std::ranges::reverse_copy(y, x.begin());
        std::cout << x << '\n';
    }

Output:

    12345 → 54321 → 12345

### See also

ranges::reverse (C++20) | reverses the order of elements in a range (algorithm function object)
reverse_copy | creates a copy of a range that is reversed (function template)

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